1. Field of the Invention
The invention relates to a voltage regulator, and more particularly to a voltage regulator with start-up circuit.
2. Description of the Related Art
A voltage regulator is designed to provide and maintain a constant voltage in electrical applications, wherein a low dropout (LDO) voltage regulator is a DC linear voltage regulator which has a very small input-output differential voltage and relatively low output noise.
FIG. 1 is a block diagram of a conventional LDO voltage regulator 100. The LDO voltage regulator 100 comprises a bandgap circuit 110, an amplifier 120, a feedback circuit 130 and a pass element 140, wherein the feedback circuit 130 has two resistors R1 and R2. An input voltage Vin is coupled to the pass element 140, and is also coupled to the bandgap circuit 110 and the amplifier 120 as a supply voltage. The bandgap circuit 110 generates a reference voltage Vref, which is a stable voltage, according to the input voltage Vin. The amplifier 120 receives the reference voltage Vref and a feedback signal VFB, and then generates a control signal Vg to control the pass element 140. The pass element 140 outputs an output voltage Vout according to the input voltage Vin and the control signal Vg. Then, the feedback circuit 130 generates the feedback signal VFB according to the output voltage Vout. Therefore, by comparing the reference voltage Vref and the feedback signal VFB, the LDO voltage regulator 100 controls the pass element 140 according to a difference between the reference voltage Vref and the feedback signal VFB to supply the output voltage Vout to a load circuit 150.
In FIG. 1, the reference voltage Vref is generated according to the input voltage Vin. For this reason, if noise interferes with the input voltage Vin, the reference voltage Vref will also be disrupted with the noise, thus varying the voltage value of the reference voltage Vref, such that the output voltage Vout does not obtain a desired voltage value. Meanwhile, the output voltage Vout is coupled to the bandgap circuit 110 as a supply voltage, to improve the performance (e.g. line regulation) of the voltage regulator, and hence the output voltage Vout is not disrupted with the noise from the input voltage Vin.
However, the output voltage Vout remains at a low voltage level when the voltage regulator is in an initial state, and the supply voltage of the bandgap circuit 110 is not high enough to individually power up the bandgap circuit 110. Thus, the reference voltage Vref will remain at a low voltage level along with the output voltage Vout. If the output voltage Vout is the supply voltage of the bandgap circuit 110, power up of the voltage regulator will be difficult. Therefore, a start-up circuit is desired to solve the above problem.